This is a bit more code and process than the other posts provided. From what I could tell, the other code doesn't actually find the latest message, just finds a message.
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Since I copied and slightly modified code from my other dbs, I may have missed something. I didn't receive errors, but it possible there may be errors if run on another system. I left code in that includes a signature. This will need to be edited or deleted. SEE NOTE ABOUT SIGNATURE in MODULE1 below before trying to run this to avoid potential error message.
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Here is a sample db to obtain the latest email sent to someone and includes the message thread. Since not sure how you are selecting the name, I am using simple text boxes. When the form opens there 3 boxes and 3 buttons.
Search After (Default is 30 days before today, can enter any date)
This is used to create a list of emails from the sent folder that were sent after that date. This only needs to be run as needed when there have been more emails added. Does not need to be run every time under the assumption that the list to search is not being updated constantly. Saves time by not having to update often.
Sent To, provide the name to search for. Can be a partial name or the whole name. This will search for any message the name appears in whether they are the only to or part of a list of recipients.
Date Sent: displays the date of the most recent email that contains the sent to name. (This date is disabled so can't be edited on the form)
Create Reply retrieves the email sent to that person on that date/time and opens it up in Reply All, so if the message was sent to more people, they will also be included. Message includes whatever text was in the original message, just as is what takes place when clicking Reply All. (Actually is searching for the date and subject rather than date and person)
NOTES
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FORM1:
I commented out the message text as not sure if you need that for your purpose since you can type a message directly in the email. If you want to use the message via code, simply uncomment it and edit the text to your message.
MODULE1:
If you use a signature, then you will need to edit the code to provide the name and location of the signature file. (Search for SigString in Module1)
If you don't need signature, can comment out the code in the appropriate places.
The Outlook_ReplyAll has a few optional parameters, for processing emails.